Hi Stefan,

Check out http://www.aade.com/filter.htm for a good & free filter design
tool. It does support coupled resonator band-pass filters also.

Coupled Resonator band-pass filters are likely to be the best match for your
requirement at the expense of high insertion loss. I don't think you'll get
better with Cauer or elliptic filters where you have to take the inductor's Q
into account at design time.

You could also add another resonator at the antenna which will provide you
with some extra gain and filtering. In that case have the greatest L/C ratio
as possible and extend the length of the whip to 1 meter . This arrangement
is sometimes referred as 'tuned whip'.

Good luck,

Michel - F5WK


Le jeudi 5 août 2010 à 15:32:45, vous écriviez :

> Dear LF,

> Currently i am setting up my active antenna for the planned LF grabber 
> here in Heidelberg. It is an active E field antenna, using a BF981 and a 
> 125 kHz signal that transforms the 137 kHz down to 12 kHz where some 
> band filtering has to be applied. Then, i need another amp stage to 
> drive the soundcards input (BF862). The high impedance of the wire input 
> is first down transformed by a BF862 stage as a source follower, then i 
> allpy a double LF bandpassfilter that is coupled by a C of some pF 
> (about 4...8 pF). This signal is applied to the 2nd Gate of the BF981...

> My question is: There may be better suited filter designs than taking a 
> L parallel C resonated at 12 kHz (after the mixing stage), between 
> signal and ground since this gives a sharp filter, ie 137,7 kHz is 
> already attenuated by 25 dB compared to 137,0 kHz. What i want to have 
> is a filter with a specific bandwith and edge frequencies with about 
> constant low attenuation in the transmission range and relative sharp 
> slopes so that 137,7 kHz is not really attenuated but 138,83 kHz 
> (DCF-39) as much as possible. DCF39 is 60 dB above noise here although 
> it gets already attenuated by the input band filter!

> Jim/M0BMU has designed a filter for his VLF loop RX that looks quite
> good. Is there a web page where i just can type the filter oder, edge 
> frequencies, input- output impedances and so on and get the values?
> I have found such one at 
> http://www-users.cs.york.ac.uk/~fisher/cgi-bin/lcfilter but i am not yet 
> experienced too much about this stuff so i am not sure if this gives 
> really useful answers.

> Any other simple ideas to come quickly to the optimal filter type, oder 
> and values? I do not want to spend too much time for that, so an 
> "excellent filter design book" is not the best hint  

> What about a cauer filter? I have read that it has the sharpest edges 
> but this may cause QRM in the pass band? (like clicks in a too sharp CW 
> filter?)

> The picture shows what i have done so far. Watching the spectrum on the 
> roof  of the institute (the future QTH) from 0...48 kHz in SpecLab looks 
> very promising so far (see picture). DLF is 60 dB above noise although 
> already attenuated about 60 dB! So filtering before mixing and further 
> amplification is necessary in my case, i assume...

> Tnx for helping ideas.

> 73, Stefan/DK7FC